1. Field of the Invention
The invention relates to semiconductor wafer processing equipment and, more particularly, the invention relates to a method and apparatus for selectively marking a semiconductor wafer with identifiable markings.
2. Description of the Background Art
Identifying defects on the surface of a wafer, such as particulate contaminants and other surface irregularities, is extremely important to integrated circuit manufacturing processes. To eliminate defect sources, defects are identified and analyzed to determine the source of the defect. Thereafter, corrective action can be taken to reduce or eliminate the defect source.
One system used to identify defects on a patterned wafer, i.e., a wafer containing dice, is the model WF-7xx manufactured by Orbot. This system uses a laser scanning technique to identify scribe lines and other features on a patterned wafer. Then, each die is scanned and compared on a gray scale level to other dice such that defects are identified as gray level differences from die to die. Such a system is very good for identifying defects on a patterned wafer. However, since the system is optimized for die scanning, it cannot be used to identify the edge location of a wafer or the notch location of a wafer. If used for edge identification the sensors would be overloaded and damaged by the high reflectivity of the wafer edge. As such, the system uses an edge exclusion process to ensure that the sensor does not scan the wafer edge.
In another prior art system, the defect identification process is accomplished in two steps. First, a laser scanner device scans a wafer with a laser and analyses the backscatter of the laser to locate defects on the surface of the wafer. One such laser scanner device is a Tencor SurfScan 6200, manufactured by Tencor Instruments. The defects are identified and logged into a defect map. The map uses a coordinate system that is relative to the center of the wafer and a notch in the wafer, i.e., the notch and wafer center form a first axis and a second axis is perpendicular to the first axis. To make all the defect coordinates fall into the first quadrant, the coordinate system is then offset by a distance equal to the radius of the wafer, i.e., each axis is tangent to the edge of the wafer. The process of using a laser within the laser scanner device to identify the wafer edge, the wafer center and the notch location provides a coordinate system that is accurate to approximately 1 mm for a blank wafer. If the wafer contains a die or dice, the scribe lines and other features on the wafer can be used to improve the coordinate system accuracy.
Second, each defect located by the scanner is analyzed to identify the root cause of the defect. The most commonly employed analysis tool is a high magnification imaging system such as a scanning electron microscope (SEM). An SEM is used to identify the defect and/or the source of the defect by inspecting the defect at high magnification. Additionally, the SEM may be accompanied by instrumentation for performing chemical analysis of the defect. Such instrumentation includes an energy dispersive x-ray (EDX) detector. Other tools may include instrumentation for performing an Auger analysis, an atomic force microscope (AFM), a tunneling electron microscope (TEM), an optical spectrometer and the like.
Since the SEM as well as other analysis tools use a high magnification (on the order of 200 to 2000 times), rapidly positioning the SEM at the defect location can be time consuming. Although the laser device provides defect coordinates that are accurate to approximately 300 microns, a substantial amount of time can be spent manually searching, for example, a 300 by 300 micron region with the SEM for a defect having dimensions of a few tenths of micron.
Additionally, when using a SEM (or other optical analysis tool) to analyze a "bare" or "blank" wafer, i.e., a wafer having no surface features, the SEM has difficulty focusing on the wafer surface. Without an accurate focus, finding a small defect on the surface is nearly impossible.
Furthermore, some defects are identified by laser scanning, but cannot be seen with a SEM. As such, the SEM operator may search for a defect for a long period of time until realizing the defect cannot be seen with the SEM.
Some defect analysis tools, e.g., an atomic force microscope (AFM), are not readily useful when a large search area is used. Generally, the field of view for an AFM is approximately 10 .mu.m.sup.2, and the search area is as large as 300 .mu.m.sup.2. Unfortunately, an AFM requires approximately five minutes to obtain a 10 .mu.m.sup.2 image. As such, 900 images are required to cover a 300 .mu.m.sup.2 search area, requiring 4500 minutes to complete the search.
Therefore, a need exists in the art for a method and apparatus that selectively marks a wafer to produce an accurate coordinate system for use by an analysis tool such that defects or any other portions of the wafer can be rapidly identified and examined with a SEM or other analysis tool.